Police are hunting a gunman believed to be behind two shootings and a carjacking in central Paris that has left one young photographer fighting for his life.

A large-scale manhunt was underway in the area around the Champs-Elysées for a man believed to be armed with a sawn off shotgun and a black sports bag containing grenades.

Police have described the suspect as a European in his early forties with a shaved head covered by a cap, wearing a long green coat and possibly a bullet-proof vest. He is said to be of medium height and heavy build.

All major news outlets in Paris were under police protection after the gunman burst into the headquarters of Libération newspaper and opened fire.

A 27-year-old man is in a critical condition after the lone intruder shot him in the chest and arm with a pump-action shotgun at 10.15am local time on Monday morning in the lobby of Libération’s offices in the 3rd arrondissement. The newspaper said the injured man worked as an assistant to one of the photographers for Next, a supplement of the newspaper.

The gunman fled the scene and was still at large several hours after the attack.

Separate reports shortly after midday said that a man also opened fire outside the towers of Societe Generale bank at Paris’ business district of La Défense. There were no injuries reported.

Francisco Alvarez a witness to the shooting at Société General in La Défense said: "I saw this guy with a cap and a shotgun, a pump-action shotgun, in his hand.”

"I don't think he was necessarily targeting anyone, he shot in the air then into a window. The first shot shocked everyone into silence and then the second caused a general panic. Then he ran away down the steps to the street,” he told AFP.

A police source said: "There are similarities in clothing, corpulence and appearance in eyewitness accounts of the man behind both attacks.”

Manuel Valls, the interior minister, said the police were scrambling to catch a “dangerous” individual.

“While he’s still at large and we don’t know his motivations, he poses a real threat. We must act fast,” he told France Info radio.

Regarding the shooting at Libération newspaper, the minister condemned a “scene of war that has nothing to do with democracy or the press”.

A journalist at Libération tweeted that the attacker had fired two to three shots on the ground floor of the premises before fleeing on foot.

"I was just arriving for work and I saw a man lying on the ground, holding his stomach and with blood everywhere," Liberation journalist Anastasia Vecrin told AFP.

François Hollande, the French president who is currently on a tour of the Middle East, said he had ordered “all possible means” to be used to find the assailant.

A helicopter was flying above the Champs Elysées early in the afternoon following reports that a man had taken a motorist hostage and forced him to drive him to the road that leads to the Arc de Triomphe. According to BFMTV, the motorist dropped the gunman off outside the Georges V, a top hotel.

A heavy police presence was reported on the Champs-Elysées but the avenue has not been cordoned off.

Subsequent reports said the man then took the metro to evade police.

The 65-year old motorist, briefly taken hostage, said his captor had shouted : "I’ve escaped from prison, take me to the Champs (Elysées) !"

The motorist obeyed and dropped him off near the avenue. Only when he got home and saw news reports about the gunman did he contact police. He was said to be suffering from shock.

Hours after the gun attack at their offices, Libération’s website suffered what it called a malicious "denial of service" attack, which blocked access to the site. The daily announced that the service was up and running again by mid-afternoon. It is not known if there was any link between this morning’s shooting and the cyber attack.

The shootings came three days after a man stormed into the Paris headquarters of news channel BFMTV and emptied several cartridges from a similar shotgun before warning a senior editor: "Next time, I will not miss you."

Police are investigating a possible link between the two incidents, and have said that CCTV footage suggests the same man might have carried out the BFMTV attack.

Liberation executive Nicolas Demorand said staff at the left-leaning newspaper founded by Jean-Paul Sartre, the philosopher, were in shock.

"When you have someone with a shotgun coming into a newspaper's offices in a democracy, it is very, very serious, whatever the mental state of the person," said Mr Demorand.

"If papers and other media have to become bunkers, something has gone wrong in our society."

He condemned a climate of rising violence in France, in particular towards journalists.